Until now myocardial infarction, angina pectoris, cerebral infarction, and so on have been considered to be the development of a stenosis on a blood vessel which perfuses organs. However, it has become evident that these diseases are caused by the susceptibility of an atheroma to rupture. The atheroma is a blood wall disease. Actually according to a clinical examination of a cholesterol-lowering agent used worldwide, the remarkable effect of improving a survival rate and preventing myocardial infarction is obtained even through the stenosis of a blood vessel hardly changes. This is because the agent stabilizes an atheroma as a tissue. In consideration of this fact, a method for examining the susceptibility of an atheroma to rupture (susceptibility to rupture) has been demanded. A conventional method such as X-ray CT, MRI, and angiography cannot carry out such an examination.
For example, in view of an accurate measurement on a blood vessel disease, conventional echocardiography M-mode just has a resolution of 1 mm. Similarly when the vibration of an aorta is determined as a displacement velocity by conventional Doppler method, conditions for accuracy are theoretically satisfied but the pulsation of a blood vessel greatly affects in reality. Thus, it is difficult to extract a small vibration superimposed to a relatively large amplitude. Hence, researchers including the inventor have developed a phased tracking method whereby a small vibration on a beating heart and a large blood vessel is remotely measured by ultrasound and an elasticity modulus of a blood vessel wall can be calculated on a given spot. Thus, it is possible to accurately diagnose the susceptibility of an atheroma to rupture (Reference Documents 1 to 5).
The following are reference documents:    1. Kanai H, Hasegawa H, Chubachi N, Koiwa Y, Tanaka M. Noninvasive evaluation of local myocardial thickening and its color-coded Imaging. IEEE transaction UFFC. 1997; 44: 752-768;    2. Hasegawa H, Kanai H, Hoshimiya N, Chubachi N, Koiwa Y. Accuracy evaluation in the measurement of a small change in the thickness of arterial walls and the measurement of elasticity of the human carotid artery. Jpn J Appl Phys 1998; 37: 3101-3105;    3. Kanai H, Koiwa Y, J. Zhang Real-time measurements of local myocardium motion and arterial wall thickening. IEEE-transaction UFFC. 1999; 46: 1229-1241;    4. Japanese Patent Laid-Open No. 10-5226; and    5. Japanese Patent Laid-Open No. 12-229078.
The phased tracking method will be schematically described below. The phased tracking method is a new bioinstrumentation for measuring a small vibration velocity on a cardiac wall and a blood vessel wall. This method makes it possible to accurately measure a vibration of 500 Hz or less and 0.01 mm and a change of 10 microns on a wall. With this method, for example, small velocities on a plurality of measurement points between the layer (or layers) in the arterial wall or on the wall of an arterial vessel are determined by ultrasonic Doppler method, and the small velocities on the measurement points are subjected to time quadrature, so that a time change in the positions of the measurement points can be calculated. Since a change in layer thickness can be determined by the time change in the positions of the measurement points, the elasticity modulus of the layer can be obtained, thereby estimating susceptibility to rupture.
Actually as shown in FIG. 17, an arterial intramural measurement point on an ultrasonic beam 91 is set at (i) and a measurement point with the subsequent depth is set at (i+1). Small vibration velocities vi(t) and vi+1(t) on the measurement points are determined and a difference between the small vibration velocities is subjected to time quadrature, so that a change Δh(t) in layer thickness between the measurement points (i) and (i+1) in the arterial wall is determined. Reference numeral 92 denotes a plaque.
      Δ    ⁢                  ⁢          h      ⁡              (        t        )              =            ∫              -        ∞            t        ⁢                  {                                            v              i                        ⁡                          (              t              )                                -                                    v                              i                +                1                                      ⁡                          (              t              )                                      }            ⁢              ⅆ        t            
The simplest method for converting the change into an elasticity value for each layer in the arterial wall is performed as follows: a wall thickness is set at hd and a change in thickness is set at Δh at the lowest blood pressure where a wall thickness increases, a pulse pressure at a cuff pressure on brachial artery is set at ΔP, and a wall elasticity modulus is measured for each layer in the following manner.
Based on a thickness change (Δh) of each layer from the intima to the adventitia of a blood vessel, an elasticity value (En) of each small part (n) in a blood vessel wall atheroma is determined by the formula below.En=(ΔP/(Δh/hd)n)
With this phased tracking method, it is possible to measure an elasticity value for each layer along the depth direction in a blood vessel wall approximately every 0.75 mm to 0.075 mm on an ultrasonic beam, thereby displaying a tomogram based on the elasticity values.
In a clinical diagnosis using the phased tracking method, when an elasticity value is examined for each layer of a blood vessel wall, the elasticity value ranges from 100 to 2 MPa in a normal person. However, in the example of an atheroma, elasticity values are not evenly distributed. It is understood that fundamentally a physically soft portion is present in a thrombus and is covered with a hard portion. In addition, various patterns are present in the tomogram of an atheroma. For example, an extremely soft substance is exposed on the lumen of a blood vessel without continuity on a capsule of a hard surface, and a substance having a large elasticity value almost entirely covers a surface. Elasticity values in an atheroma are distributed from 0 to 4 MPa. Further, according to the examination results on the correspondence of a lipid-rich layer and a collage fiber in an extracted blood vessel and elasticity values in the phased tracking method, there is a probability that the tissue image of a collagen fiber covering an atheroma can be separately displayed from a tomogram of elasticity values, the tomogram being obtained in a noninvasive manner.
It is an object of the present invention to provide more specific information displayed in a tomogram and to identify the kind of living tissue such as a lipid-rich area, a thrombus area, an elastic fiber, a collagen fiber, and a calcified area in an ultrasonic diagnosis of a living tissue such as a lesion on a blood vessel.
It is another object of the present invention to identify the kind of living tissue such as a lipid-rich area, a thrombus area, an elastic fiber, a collagen fiber, and a calcified area by using a shear elasticity modulus and a shear viscosity in an ultrasonic diagnosis of a living tissue such as a lesion on a blood vessel.